April
12, 2004 — A NOAA researcher is
using NOAA satellite technology
to take a closer look at the effect African air may have on hurricanes,
especially how they develop and change intensity. Forecasting this change
would provide forecasters, emergency managers and citizens the information
they need to determine appropriate action before a storm makes landfall.
(Click NOAA satellite image for larger view of Saharan Air Layer
or SAL. Click here for high resolution version,
which is a large file. Please credit “NOAA.”)

Jason Dunion,
a research meteorologist from the NOAA Cooperative Institute for Marine
and Atmospheric Studies (CIMAS),
working with the NOAA Hurricane
Research Division in Miami, Fla., describes a new use of satellite
technology to track and describe the role of the Saharan Air Layer (SAL)
in Atlantic hurricane development and intensity
change. Intensity change is an increase or decrease in wind speed
consistently for at least one minute at an altitude of about 30 feet.
Dunion’s work appears in the current (March) issue of the Bulletin
of the American Meteorological Society.

The SAL is
an elevated layer of hot, dry, dust-laden African air that often covers
a large area of the tropical Atlantic during the hurricane season. The
SAL has been investigated for several decades but its link to Atlantic
hurricane activity had never been fully examined.

The SAL is
not readily detectable with the individual visible and infrared (IR) sensors
on most geostationary (GOES)
and polar orbiting environmental satellites (POES).
Dunion’s work discusses recently developed GOES split-window satellite
imagery that permits continuous tracking of the SAL across the North Atlantic,
Caribbean and Gulf of Mexico during the late spring through early fall,
coincident with the Atlantic hurricane season (June 1 – Nov. 30).

“The
SAL’s influence on hurricanes may be a factor in the challenge forecasters
face in accurately predicting a tropical cyclone’s intensity in
the Atlantic,” said Dunion. “It also may contribute to this
basin’s relatively reduced level of hurricane activity compared
to other ocean basins.”

The SAL is
formed from a deep, well-mixed, dry (adiabatic) layer of air that forms
over the Sahara Desert and Sahel regions of North Africa from late spring
to early fall. As this air mass moves west and emerges from the northwest
African coast, it is undercut by cool, moist low-level air and becomes
the Saharan Air Layer. The SAL contains very dry air and substantial mineral
dust lifted from the arid desert surface over North Africa and is often
associated with a mid-level easterly jet stream.

Using Dunion’s
multi-spectral GOES infrared satellite technique, forecasters can follow
the progress of the SAL as it moves west over the tropical Atlantic. This
imagery shows that when the SAL engulfs tropical waves, tropical disturbances
or pre-existing hurricanes, its dry air, temperature inversion and strong
vertical wind shear (associated with the mid-level easterly jet) can inhibit
hurricane intensification.

“We
are intrigued by this and other science that relates to the Saharan Air
Layer and its potential to help advance our skill in forecasting tropical
cyclone intensity,” said Max Mayfield, director of the NOAA
National Hurricane Center.

The most
efficient way to study and quantify the effects of the SAL’s dry
air and strong mid-level jet is by first identifying it with the GOES
SAL tracking imagery and then directing “hurricane hunter”
aircraft to fly through it to determine its structure. The NOAA Gulfstream
G-IV high-altitude hurricane surveillance aircraft releases instrument
packages called Global Positioning System dropwindsondes in the area around
the hurricane. Data collected in hurricanes from 1998-2001 have provided
insight into the processes by which the SAL affects tropical disturbances.

“The
GOES technique developed by Jason Dunion will provide us with a much better
picture of the impact of the SAL and African dust on the meteorology of
the tropical Atlantic and Caribbean and its role in tropical storm development,”
said Prospero.

CIMAS is
a joint research institute between the University of Miami’s Rosenstiel
School and the NOAA Atlantic Oceanographic and Meteorological Laboratory
in Miami, Fla.

The American
Meteorological Society is the nation’s leading professional society
for scientists in the atmospheric and related sciences.

NOAA is dedicated
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and research of weather and climate-related events and providing environmental
stewardship of the nationís coastal and marine resources. NOAA is part
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